Biome related content will generate in unexplored areas. So if you have an old world with mostly explored areas, then it's prudent to start a new world. However, if you didn't go too far and would like to keep your old stuff, then continue to play with your old world.

Mind you, Mistlands will be the next major update. So any Mistlands you already found, will not be populated with the new content when it comes out.